Re-add read_only_adaptor::move_at specialisation

Since #97 this was falling back to `map_adaptor::move_at()`, which does the wrong thing in C++23 mode when the base element type is something like `tuple<T&>`.

Fixes #117

include/flux/op/read_only.hpp

@@ -30,8 +30,25 @@ struct read_only_adaptor : map_adaptor<Base, cast_to_const<const_element_t<Base>
     {}
 
     struct flux_sequence_traits : map::flux_sequence_traits {
+    private:
+        using const_rvalue_element_t = std::common_reference_t<
+            value_t<Base> const&&, rvalue_element_t<Base>>;
+
+    public:
         using value_type = value_t<Base>;
 
+        static constexpr auto move_at(auto& self, cursor_t<Base> const& cur)
+            -> const_rvalue_element_t
+        {
+            return static_cast<const_rvalue_element_t>(flux::move_at(self.base(), cur));
+        }
+
+        static constexpr auto move_at_unchecked(auto& self, cursor_t<Base> const& cur)
+            -> const_rvalue_element_t
+        {
+            return static_cast<const_rvalue_element_t>(flux::move_at_unchecked(self.base(), cur));
+        }
+
         static constexpr auto data(auto& self)
             requires contiguous_sequence<Base>
         {